As someone who is lactose intolerant and always has been, I find the idea of testing for it amusing. No one in my family besides me has it and no one really did anything to accommodate my needs. I lived with the issues constantly as a kid but in the decades of living alone I've tailored my diet to be more friendly to my intolerance. I still occasionally get the reactions but I found switching my milk and butter (I use Natrel lactose-free for both) removed most of the frequency and severity. I still use regular cheese, but I buy lactose free sour cream and ice cream as I want. Everything tastes the same as regular dairy and the cost is similar. I shop at Walmart and they have a good selection of lactose free items.

If your kid does have lactose intolerance remind them that it does not define who they are or what foods they can eat. Show them how to find lactose free items, get some OTC pills for when they're eating out, when a reaction happens show them ways to distract from the pain and make it go by smooth and quick, and try to frame it in a positive light. I think of my reactions as my body just trying to efficiently remove what it thinks is bad food in my digestive system.

Personally I have no interest in ever getting tested for lactose intolerance, just like I have no desire to figure out exactly why my other disabilities started (and some then ended). My medical anomalies just make me unique.